The present invention relates to an optical dispersion compensator that is suitable for application in an optical transmission system using optical fibers, or in a system in which an optical transmission scheme based on wavelength multiplexing has been adopted.
In recent years, there has been increased activity in the study and commercialization of a long distance optical transmission system, in which an optical amplifier is used as a repeater. In order to cope with multi-media service, with a particular emphasis on the internet, a trend toward larger capacity with use of WDM (Wavelength Division Multiplex) for multiplexing multiple light signals having different wavelengths to one piece of optical fiber, which serves as a communication transmission path, has been considered as an effective technique. In such a long distance optical transmission system, the transmission speed or transmission distance is restricted to a large extent by a phenomenon referred to as the wavelength dispersion of the optical fiber.
Wavelength dispersion is a phenomenon in which light having different wavelengths propagates through an optical fiber at different speeds. Since the optical spectrum of an optical signal that is modulated at high speeds includes different wavelength components, these components reach a receiver at different times under the influence of wavelength dispersion when propagating through optical fibers. As a result, distortion occurs in the optical signal waveform that is received after optical fiber transmission.
In order to suppress the generation of waveform degradation as a result of such dispersion, a technique referred to as dispersion compensation has been employed. Dispersion compensation is a technique for canceling out the wavelength dispersion characteristic of the optical fibers, so as to prevent the waveform from being degraded, by arranging an optical element, which has the inverse of the wavelength dispersion characteristic of the optical fibers being used as the transmission path, in an optical transmitter, a receiver, a repeater or the like. As an optical element, that is, a dispersion compensator, devices having an inverted dispersion characteristic, such as dispersion compensation fibers and an optical fiber Bragg grating, have been studied and commercialized.
The dispersion tolerance represents a range of residual dispersion (total sum of amounts of dispersion by transmission path fibers and the dispersion compensator) that satisfies a certain reference of transmission quality. Since the dispersion tolerance becomes smaller inversely with the square of the bit rate of an optical signal, the dispersion compensation technique becomes even more important as the transmission speed rises. For example, in a 10 Gbps transmission system, when it is considered that the dispersion tolerance of the optical signal is nearly 1000 ps/nm, and that the amount of dispersion of a single mode fiber is about 17 ps/nm/km, the system will be able to transmit only about 60 km without the aid of the dispersion compensation technique. On the other hand, the dispersion tolerance in a 40 Gbps transmission is about 60 ps/nm, that corresponds to {fraction (1/16)}, and this corresponds to about 4 km in a single mode fiber.
At present, since the transmission distance of the main line stem optical fiber transmission using an optical repeater is from several tens of kilometers to about thousands of kilometers, it is necessary to select the amount of dispersion of the dispersion compensator in accordance with the transmission distance. For example, in a 10 Gbps transmission system, a method has been adopted in which a dispersion compensator having a fixed amount of compensation in increments of 100 ps to about several hundreds ps is provided in advance in consideration of the dispersion tolerance, and the amount of compensation is determined in accordance with the transmission distance at the time of installation for the given installation, or the like. In this case, for a dispersion compensator, there is adopted a representative method for using a dispersion compensation fiber having wavelength dispersion with an inverse symbol relative to the transmission path.
In a 40 Gbps transmission system, a dispersion compensator whose amount of compensation dispersion is capable of varying in increments of 10 ps to about several tens of ps similarly is considered necessary. Moreover, in this case, variations in the amount of wavelength dispersion due to the temperature of the transmission path fibers cannot be ignored. For this reason, a dispersion compensator that is capable of controlling the amount of dispersion in a variable manner has become necessary.
However, these conventional dispersion compensators also have various problems. When dispersion compensating by a fixed amount, since huge compensation fibers as long as several kilometers to hundreds of kilometers are required for the dispersion compensation, the required storage space for the fibers becomes large. Also, in order to compensate for signal losses in the dispersion compensation fibers, there is the possibility that extra light amplifiers will be required. Further, the dispersion compensation fibers generally have a small mode field diameter, and there is the possibility that they cause a great fiber non-linear effect and distortion of the transmission waveform.
In the case of an optical fiber Bragg grating, the compensation characteristic greatly changes depending upon a slight change in the wavelength because ripples exist on the wavelength in the transmission characteristic and the wavelength dispersion characteristic. Accordingly, it is known that the transmission characteristic thereof, when used for dispersion compensation, is inferior to that of the dispersion compensation fibers. Also, those having a large amount of dispersion and a large wavelength bandwidth are difficult to fabricate from a production viewpoint, and there is a problem that those having a narrow bandwidth require stabilization in both temperature and wavelength. Also, in the dispersion compensation fibers, the amount of dispersion cannot be continuously made variable in principle, and so it is difficult to realize such a variable dispersion compensation, since the amount of dispersion is being continuously changed in accordance with the change in the amount of dispersion in the transmission path.
In the case of an optical fiber Bragg grating, as a method of realizing continuous variable dispersion compensation, there has been reported, for example, a method for producing a chirped grating by giving a temperature gradient in the longitudinal direction of the optical fiber Bragg grating for effecting dispersion compensation transmission. In this case, by controlling the temperature gradient, it becomes possible to dispersion compensate for a variable amount. However, this method makes it difficult to obtain a uniform temperature gradient, and it has problems in that a sufficient dispersion compensation of performance cannot be attained, such as due to ripples occurring in the wavelength dispersion. This poses a problem in actual practice.
Therefore, it is an object of the present invention to solve such problems as described above and to provide a dispersion compensator which has hardly any ripples in the broad bandwidth.
The structure is arranged such that an interferometer, the reflection factor of one surface of which is nearly 100%, and a mirror are arranged in parallel or at a slight angle, whereby light emerging from the first collimator becomes incident on the second collimator after resonance and emission are repeated two or more times by the interferometer. Further, through the use of an interferometer, in which at least one of the reflection factor or the thickness changes in the longitudinal direction, the interferometer is caused to slide or its temperature is changed by a heater or the like, whereby there is provided an arrangement in which the amount of dispersion is variable for the dispersion compensator. Further, these dispersion compensators are provided in a multistage arrangement, whereby it becomes possible to realize a variable dispersion compensator that is applicable to a wavelength multiplex optical transmission system whose ripples have been restrained in the broad bandwidth. A more detailed description of exemplary embodiments will be set forth in the following more detailed description of the present invention.